The present invention relates in general to sensor assemblies and in particular to a sensor assembly including a piezoresistive membrane for detecting he tilt and vibration of a body and for generating output signals indicative of the tilt and vibration detection.
Today""s automobiles and similar vehicles typically include highly sophisticated and complex systems and rely on control systems that receive input signals from a variety of sensing devices. Automobiles and similar vehicles powered by internal combustion engines depend on lubricating fluids such as oil for lubricating moving components of the engine and to help maintain the engine at a proper operating temperature. Oil refining and formulation itself has become a complex process to ensure that internal combustion engines are properly lubricated and cooled during operation while taking into account environmental conditions. It is well known that oil quality varies and that over time the level of oil within an engine may be reduced due to leaks and/or oil combustion and the condition of oil deteriorates. Consequently, numerous types of electronic sensors and sensor assemblies have been developed to monitor and/or detect oil level, oil temperature, and oil condition, for example. These sensors and assemblies may include algorithms that depend on determining whether a vehicle is parked on an incline and/or whether a vehicle""s engine is running or shutdown. Oil level measuring algorithms are known in the art. Some are known to use averaging techniques to dampen the affect of an automobile being parked on an incline when the oil level measurement is taken. Other algorithms or methods may only take an oil level measurement when it is known that a vehicle is parked on a level surface.
Many tilt sensors are of the electrolytic type. A typical electrolytic sensor includes a glass or ceramic envelope that is partially filled with a conductive fluid. The fluid moves in response to tilting of the sensor where the fluid is under the influence of gravity, such as with a carpenter""s spirit level. In other embodiments the fluid may be under the influence of the acceleration of a body. Platinum contacts may be sealed flush with the inside walls of the envelope. When such a sensor is at its zero position the electrical impedance of the fluid from the center electrode to each of the left and right electrodes is equal. Tilting the sensor disturbs this balanced condition and the impedance changes in proportion to the tilt angle. Cost and size of a typical electrolytic sensor limit their use in certain environments. Many electrolytic sensors are sensitive to temperature change and temperature compensation needs to be provided in most of the signal conditioning electronic units. Also, with respect to glass electrolytic sensors, great care must be afforded to the thermal and mechanical stress related characteristics of glass during installation and alignment. This may limit the range of applications of such sensors.
Another known category of tilt sensor types is switch sensors, which may be a variation of an electrolytic sensor. A switch type tilt sensor doesn""t use a linear output with respect to inclination angle. Instead, a signal is generated once the inclination reaches a predetermined threshold.
While not used specifically for detecting the tilt of a body, a common structure for measuring pressure are pressure transducers. Pressure transducers may be diaphragm-based transducers that convert an applied pressure into stresses in the plane of the diaphragm. The stresses may be measured and converted into electrical signals by use of piezoresistive sensors that are an integral part of the diaphragm. Depending on the application, the diaphragm may be fabricated of metal or a semiconductor material such as silicon. Such configurations are known to be used in microphones, the automotive industry such as for checking tire, gas and air pressure, the biomedical industry such as for determining blood and fluid pressure, various instrumentation and vacuum sensing.
A known configuration for such an arrangement may be semiconductor devices with the resistive bridge legs formed such as by appropriate doping of selected portions of material in the semiconductor material. For example, U.S. Pat. No. 5,614,678 discloses a device using semiconductor material that is lightly doped N- or P-type silicon in a portion of the crystallographic plane. The piezoresistive elements of that device may be fabricated from P+ or P++ silicon in the crystallographic plane using known techniques. The piezoresistive sensing elements may be arranged in a Wheatstone bridge circuit so that two piezoresistors are positive changing and the other two are negative changing. The disclosed arrangement allows for an output voltage to be generated, which is indicative of an applied pressure on the device.
The condition of the lubricant such as the oil of an internal combustion engine is an important factor in determining whether fresh lubricant needs to be added to a system or whether the lubricant needs to be changed. Some such algorithms or methods may depend on an ignition signal to determine whether the engine is running or shutdown. Ignition signals are not always indicative of whether the engine is actually running or shutdown and may thereby adversely affect the oil condition algorithm or method.
Considering the cost and size advantages of using a semiconductor sensor relative to known tilt sensor types such as an electrolytic sensor, for example, it would be advantageous to provide a low cost semiconductor sensor for detecting tilt that could be adapted for a range of environments such as those requiring small scales in size. It would also be advantageous to provide a senor for accurately determining whether an automobile""s engine was running or shutdown.
An exemplary embodiment of an apparatus in accordance with one aspect of the present invention allows for the detection of certain operational parameters associated with a structure or a vehicle such as an automobile. For example, one exemplary embodiment may include an apparatus for detecting the tilt of the automobile relative to a reference position such as horizontal and vibration of the automobile. Vibration of the automobile may be used to determine whether the automobile""s engine is running, e.g., the automobile has been started and the engine is idling or generating higher revolutions per minute (xe2x80x9cRPMxe2x80x9d) than that of idling.
One exemplary embodiment of the present invention allows for electronic data indicative of engine running and vehicle tilt to be output from the apparatus and transmitted to a microcontroller or processor of the automobile for data analysis. The microcontroller or processor may send electronic signals indicative of output from the analyzed data to a display device such as a driver information center of an automobile, for example.
One exemplary embodiment for detecting tilt and vibration of a body in accordance with one aspect of the present invention takes advantage of the crystallographic structure and piezoresistive effect in semiconductor materials. The body may be a commercial or private vehicle, for example, or it could be any other physical structure where detecting tilt and/or vibration is desired. Such an apparatus may include a membrane where a plurality of piezoresistors may be diffused onto the membrane. A weight may be integral to the membrane and placed near the membrane""s center. In one exemplary embodiment four piezoresistors are diffused onto or formed integral with the membrane to form a conventional Wheatstone bridge circuit. A constant voltage input may be applied to the membrane. A change in resistance of one or more of the piezoresistors may be used to create a first voltage output signal and a second voltage output signal by means of the Wheatstone bridge. The first voltage output signal may be responsive to the angular displacement or tilt of the membrane from a reference position such as horizontal. For example, the membrane may be formed within a substrate that is affixed to a body such as an automobile. When the automobile is tilted relative to horizontal the apparatus will generate an output signal indicative of the tilt. The second voltage output signal may be responsive to vibration of a body such as the vibration of an automobile caused by the automobile""s engine running. The voltage outputs at any point in time from the Wheatstone bridge may be indicative of the stresses incurred by the membrane due to the membrane""s tilting or sensation of vibration. This may allow for a qualitative or quantitative measurement of the membrane""s angular displacement or tilt.
In one exemplary embodiment of the present invention two piezoresistors may be placed within the membrane perpendicularly or transverse to an applied stress and the other two may be placed parallel or longitudinally to the applied stress. The piezoresistors may be placed in locations that allow for maximum sensitivity of the sensors. This may be at or near the edges of the membrane where maximum stress occurs under an applied pressure. Sensor sensitivity may be defined as the ratio of change of voltage output to applied pressure. With no stress applied to the membrane in this configuration the voltage output is zero. As the applied pressure or stress is increased the voltage output will increase allowing for a determination of the membrane""s angular displacement or tilt. One advantage of this configuration is that resistance changes resulting from temperature variations tend to cancel each other. In this respect, the sensor is at least partially immune to the effects of temperature.
The membrane according to one aspect of the present invention may be selected to have an appropriate thickness so that at least a portion of it will deflect in response to an applied pressure caused by a weight proximate the center of the membrane when the membrane is tilted. The deflection may stress the piezoresistors thereby changing their respective resistance value. A Wheatstone bridge circuit may be used to transform the changes in resistance value into the first voltage output, which may be indicative of the amount the membrane has tilted, and the second voltage output, which may be indicative of vibration of the automobile caused by its engine running. The composition of at least a portion of the membrane may be substantially of silicon or a silicon-based alloy provided that electrical isolation is achieved among the piezoresistors. The weight may be composed of a variety of suitable materials such as copper or nickel, for example, and be affixed or secured to the membrane by known techniques such as bonding or electrodeposition, for example.
One advantage of the apparatus or sensor in accordance with the present invention is that exemplary embodiments may be fabricated in varying sizes. Empirical simulation using commercially available finite element analysis software such as software from Ansys, Inc. demonstrates that the dimensions of the membrane and weight vary in proportion to one another and may consequently be adapted for a wide range of applications requiring specific size sensors.
Extending the life cycle of a vehicle""s lubricant, such as motor oil, is important to today""s commercial and private motorists. Monitoring lubricant level in an oil pan, for example, is an important indicator of whether new lubricant needs to be added or the existing lubricant needs to be changed. Taking single lubricant level readings may be misleading due to the angular displacement or tilt of the vehicle, the time after engine shutdown at which the reading is taken, lubricant temperature, sloshing of lubricant within the vehicle and a capillary effect that may delay lubricant return to an engine oil pan. Also, accurate level readings may be difficult if engine shutdown is determined by a microcontroller when the ignition signal feed to the level sensor switches from xe2x80x9c1xe2x80x9d to xe2x80x9c0xe2x80x9d, for example. A problem with this approach is that the ignition signal used may remain in the xe2x80x9c1xe2x80x9d state even after the engine is shutdown in order to allow certain systems of an automobile to continue working after the ignition key is turned to the xe2x80x9coffxe2x80x9d position. Similarly, the ignition signal may be xe2x80x9conxe2x80x9d when the key is in the accessory position. These situations could lead to a reduced number of level readings. One advantage of the present invention is that an accurate measurement of a vehicle""s tilt can be made during and after engine shutdown and an oil level-measuring algorithm if desired may use this information.
Using ignition signal feeds to the sensor assembly may also adversely impact an oil condition trending algorithm, for example, in two ways. First, the number of real oil condition trend (xe2x80x9cOCTxe2x80x9d) points may be reduced because the data for generating the OCT points is collected at predetermined intervals after engine shutdown. If the sensor assembly collecting the data for the OCT points determines shutdown by the ignition signal feed there will be an inherent delay in when the sensor will begin collecting data. The second adverse impact is that the fail-safe approach for odd conductivity behavior of lubricant could be thwarted. This fail-safe approach may assume that hours of ignition xe2x80x9conxe2x80x9d are equivalent to hours of engine running, which would not be the case with the key in the accessory position. If the OCT points establish a trend that deviates from accepted performance specifications then a vehicle""s main computer program may generate a change-oil-soon or a change-oil-now command. If one of these commands is triggered too soon then the oil of the vehicle may be prematurely changed.
An oil condition trending algorithm and/or an oil level-measuring algorithm, such as ones developed by the assignee of the present invention, stored within a processor of the automobile, may use the information generated by an exemplary embodiment of the present invention to overcome the difficulties noted above.